The present invention relates to a roll for a paper or board machine, and in particular to a roll of the type having a number of openings extending through the mantle.
In paper or board machines, a web forming section employs mainly suction rolls which usually comprise a perforated roll mantle attached to end flanges at the ends of the roll. The end flanges are in turn journalled rotatably on attachment flanges situated at the ends of the roll and attached to the frame of the machine. Inside the roll mantle, there may be a static suction box attached to the attachment flanges enabling suction to be applied to a given sector of the suction roll. The interior of the roll may also be empty, in which case suction is applied to the entire circumference of the roll mantle. The ends of the roll are provided with ducts by which an external source of negative pressure can be connected to the roll. Moreover, bores extending through the roll mantle are normally provided, in the outer surface of the mantle, with countersinks by means of which the unbroken connecting portions surrounding the holes of the bores in the outer surface of the roll mantle are made smaller and the open area of the outer surface of the roll mantle is increased.
The press section of paper or board machines in turn employs rolls which have a roll mantle that is perforated or provided with blinded bores. In that case, the interior of the roll is not necessarily connected to a separate source of negative pressure. In a press nip, water is sucked into the holes, blind-drilled bores or other recesses of the roll mantle and removed from them after the press nip by means of the centrifugal force. In order to reduce the contact pressure, the mantle of press section rolls is normally coated with a material that is softer than steel, for example, with some rubber-like material. The blind-drilled bores in a roll provided with a coated mantle may extend some distance into the steel mantle or merely into the coating depending on a desired volume of the bores. Moreover, both through bores and blind-drilled bores are normally provided with countersinks in the outer surface of the mantle for reducing the size of the unbroken connecting portions that surround the holes or recesses in the outer surface of the roll mantle and for enlarging the open area of the outer surface of the roll mantle.
Around the perforations of the roll mantle on the outer surface of the roll mantle, despite holes, blind-drilled bores or recesses, there remain relatively large unbroken connecting portions at which the suction effect is weaker. For this reason, said unbroken outer surface of the roll mantle causes marking in the paper web.
One solution to his marking problem has been to provide the roll mantle, for example, with a coarse wire net, by which the open surface of the outer face of the roll mantle has been increased. The wire net or a wire sock is mostly made of plastic and it is attached in place by shrinking to form the outermost layer of the roll. The manufacture of such a wire sock and its fitting to the outer face of the roll mantle constitute an additional work stage in the manufacture of the roll. In addition, the wire sock wears in use and thus it has to be replaced at certain intervals.
It is also known to mount on the roll mantle a separate honeycomb arrangement made of metal by means of which the open surface of the outer face of the roll mantle is enlarged. It is difficult to fasten this kind of metal honeycomb to the face of the roll mantle and it may become detached in use.
DE patent 21 40 776 discloses a suction roll of a paper machine comprising a mantle stiffened against bending and a perforation extending through the mantle of the roll and forming a certain pattern. Additionally, the mantle surface of the roll is provided with grooves that connect a row of holes so that a symmetrical embossed pattern of the surface is formed in practice. The hole area in the surface of the roll mantle is over 50% and it may be nearly 90% of the total area of the roll mantle. It is also stated in the publication that some of the above-mentioned holes may be blind-drilled bores or that, in addition to the above-mentioned holes, blind-drilled bores are made into the surface of the mantle for improving the water retention capacity of the roll.
In this arrangement, the connecting surface of the walls between two adjacent grooves in the surface of the mantle forms a solid connecting portion supporting the wire or equivalent.
The problem in this arrangement of DE patent 21 40 776 is the solid connecting portions at which the suction effect of the roll is weaker. These solid connecting portions constitute an obstruction to the free flow of water into the holes or blind-drilled bores.
The arrangement in accordance with the invention provides an essential improvement over the prior art arrangements.
The roll in accordance with the invention provides a very good and even flow of water into the holes extending through the mantle of the roll and/or into the blind-drilled bores and/or equivalent openings situated in the outer surface of the roll mantle. Moreover, in the roll in accordance with the invention, no separate wire sock is needed on the outer surface of the roll mantle. The open area of the outer surface of the mantle of the roll in accordance with the invention is about 70-90% depending on the application.
The arrangement of the invention may be used in a roll of a paper or board machine which comprises either openings extending through the roll mantle, e.g. through bores, or recesses formed into the outer surface of the mantle, e.g. blind-drilled bores, or a combination of them. Such rolls are used, for example, in a web former and in a press section. The invention may be used in a suction roll where suction is applied to the circumference of the entire mantle or in a suction roll having a static suction box by means of which suction is applied to a given sector of the roll. The arrangement in accordance with the invention may also be used in a roll which employs no external source of negative pressure, by which a negative pressure is maintained in the interior of the roll. In that case, the water that is being removed from the web is transferred into the holes and/or blind-drilled bores of the roll mantle at the point of compression by the action of a pressure difference produced in the wire or equivalent supporting the web.